503COFFERDAMS AND WATER DIVERSION STRUCTURES

New Hampshire Department of Transportation Standard Specifications – 2016 Description

1.1This work shall consist of the design, construction, maintenance, and re moval of all cofferdams, caissons, cribs, sheeting,

embankments, channel diversion structures, pipes, and other similar work, including dewatering, required to allow the excavation for foundation units and to permit and protect the construction of bridge or other structural units. Materials

2.1All materials to be used in the construction of this work shall be subject to inspection and approval prior to their incorporation

in the structure.

2.2Sheeting specified to be left in place shall meet the requirements of 506.2.

Construction Requirements

3.1General.

3.1.1 Cofferdams and water diversion structures shall be constructed so as to protect fresh concrete against damage from a sudden rising of the stream and to prevent damage to the footings and bridge units by erosion. 3.1.2 The Contractor shall submit drawings showing materials to be used, proposed method of construction, and other details left open to choice or not fully shown on the plans. When cofferdams are required or proposed, such information shall show the arrangement for flooding the cofferdam. Drawings shall be submitted for documentation in accordance with 105.02 .

3.2Water Diversion Structures.

3.2.1 Temporary diversion channels, dikes, embankments and other similar structures shall be constructed in a way to minimize erosion and resulting water pollution. Appropriate measures of protection shall be incorporated into their construction. 3.2.2 Pipes, sluices, elbows, etc. that are used in the diversion of water shall be of sufficient size to carry the flow specified on the plans. 3.2.3 Temporary diversion channels, dikes and embankments shall be completely removed and restored to original or design grades as directed by the Engineer.

3.3Cofferdams.

3.3.1 Cofferdams or cribs for foundation construction shall be carried to adequate depths and heights, shall be safely designed and constructed, and shall be as watertight as necessary for the proper performance of the work which must be done inside them. Interior dimensions shall be such as to give sufficient clearance for the construction and exterior inspection of forms and to permit pumping outside of the forms. In no case shall the sheeting be placed inside the neat lines shown for the concrete footing or seal. No construction material shall be left in cofferdams in such a way as to extend into the substructure masonry, without permission. 3.3.2 No excavation shall be made outside of cofferdams, nor shall the existing streambed adjacent to the structure be disturbed without the approval of the Engineer. When the Contractor is permitted to make excavation at the site of the structure before cofferdams are placed, all openings outside the cofferdams shall be backfilled with approved material to the elevations of the original ground or streambed after the cofferdams have been placed. 3.3.3 Cofferdams which are tilted or moved laterally during the process of sinking shall be righted, reset, or enlarged, so as to provide the necessary clearance, at the Contractor’s expense. 3.3.4 Unless otherwise provided, cofferdams including all bracing shall be removed by the Contractor after the completion of the substructure, with care being taken not to disturb the surrounding soil or injure the finished masonry. 3.3.5 Concrete foundation seals, when required or permitted, shall be constructed as provided in Section 520. 3.3.6 Cofferdams with sheeting left in place. 3.3.6.1 When called for on the plans, cofferdam sheeting shall be cut off at the top of the foundation seal or footing or at the specified elevation. The length of sheeting left in place below the specified cut off elevation will be the minimum embedment length shown on the plans or the length required on the approved cofferdam design drawings. 3.3.6.2 Anchorage between the cofferdam sheeting and concrete foundation, if required, will be detailed on the plans.

3.4Dewatering.

3.4.1 Control of water within cofferda ms shall be in such a manner as to prevent disturbance of the bearing soil, or fresh concrete, or segregation of backfill material. Pumping areas shall be properly filtered to prevent pumping of fines.

New Hampshire Department of Transportation Standard Specifications – 2016 3.4.2 No pumping shall be permitted during the placing of concrete or for a period of at least 24 hours thereafter, unless it is done from a suitable sump separated from the concrete work by a watertight wall. 3.4.3 Pumping to dewater a cofferdam with a concrete foundation seal shall not commence until the s eal has set sufficiently to withstand the hydrostatic pressure, and in no case before 5 days have elapsed following the placing of the concrete. 3.4.4 Any foundation soil weakened as a result of insufficient care taken in maintaining a dewatered condition shall be removed and replaced with structural fill at the expense of the Contractor. Method of Measurement

4.1The accepted quantity of water diversion structures, cofferdams, and cofferdams with sheeting left in place will each be

measured as a unit. When more than one unit is specified in the Contract, separate item numbers will appear for each separate and complete unit. Basis of Payment

5.1The accepted quantity of water diversion structures, cofferdams, and cofferdams with sheeting left in place will be paid for

at the Contract unit price. The cost of any required anchorages shall be subsidiary.

5.2When no quantities for these items are included in the proposal, the work shall be subsidiary except as may be provided under

5.3.

5.3Alterations.

5.3.1 When the foundation is to be placed on earth, the cost of necessary alterations to the cofferdams due to required changes of more than 1 ft. in depth of the bridge excavation will be paid for as provided for in 109.04. 5.3.2 When the foundation is to set on existing rock as prepared in accordance with 504.3.2, the cost of necessary alterations to the cofferdams when existing rock is encountered at an elevation more than 5 ft. below that anticipated at the time of the proposal will be paid for as provided for in 109.04 . In cases where the plans show a sloping rock surface, no extra payment will be made unless the depth to rock is more than 5 ft. below the lowest elevation shown. When Extra Work is ordered, the cost of the first 5 ft. of additional depth of the cofferdam will not be included in the Extra Work payment. If rock is ordered removed in addition to that required by 504.3.2, the cost of any necessary alterations to the cofferdams resulting from such rock removal will be paid as provided for in 109.04. Pay items and units: 503.1____ Water Diversion Structures Unit 503.2____ Cofferdams Unit 503.3____ Cofferdams with Sheeting Left in Place Unit

New Hampshire Department of Transportation Standard Specifications – 2016

Description

This work shall consist of the excavation, satisfactory disposal, and backfill of all materials encountered in the construction

of bridges as shown on the plans or ordered. Classification of Materials

Common bridge excavation shall consist of all materials not classified as rock. Glacial till or boulder clay will be considered

as common bridge excavation.

Rock bridge excavation shall consist of solid rock which cannot be removed without blasting or ripping. It shall also consist

of boulders and parts of masonry structures when found to measure 1/2 cy or more. Construction Requirements

Preparation of a Foundation on Earth.

3.1.1 In areas of excavation where pumping is not required, material shall be removed and maintained a minimum of 1 ft. outside the neat lines of the footing. 3.1.2 In areas of excavation where pumping is required, material shall be removed and maintained to such limits as will provide for carrying water outside all footing forms, with the water level maintained slightly lower than the elevation of the bottom of all footings. Suitable sumps shall be constructed and maintained. 3.1.2.1 Where materials are encountered which are susceptible to softening or displacement when inundated, pumping shall begin prior to removal of material to grade and shall continue on a 24-hour basis until masonry has been placed. 3.1.2.2 When blasting is required, the provisions of 203.3.2 and 203.3.3 shall apply as though contained in this Section. 3.1.3 The final removal of the foundation material to grade shall be made in such a manner that the foundation material below grade will not be disturbed or loosened and a uniform foundation will be obtained. Removal shall be made by hand unless the use of power equipment for this pur pose performs satisfactorily. This portion of the excavation shall be made in as short a time as possible before the masonry is to be placed. 3.1.4 When ordered, a layer of structural fill of sufficient depth to stabilize the foundation material shall be applied prior to placing either forms, reinforcing steel, or masonry.

Preparation of a Foundation on Rock.

3.2.1 Care shall be taken to avoid undue overbreakage in the drilling and blasting of rock. 3.2.1.1 When blasting is required, the provisions 203.3.2 shall apply as though contained in this Section. 3.2.2 The surfaces to receive the footings shall be level, stepped, roughened, doweled, or any combination thereof as directed. When the use of dowels is ordered, holes shall be drilled to the depth required and the dowels grouted with cement mortar conforming to Section 707. The surfaces shall be cleaned and maintained clean until the masonry is placed. 3.2.3 All loose rock and fragments shall be removed as directed. 3.2.4 Seams shall be cleaned and grouted when ordered.

Inspection. After each excavation is completed, and prior to placing forms for concrete, the Contractor shall secure approval

of the depth of excavation and the character of the foundation material.

Backfill.

3.4.1 Where masonry is to rest on a rock surface, all space resulting from excavating rock within vertical planes through the neat lines of the footings shall be backfilled with concrete of the same class as that in the footings, unless otherwise shown or ordered. 3.4.2 All spaces resulting from excavation adjacent to the structure shall be backfilled to the level of the surrounding ground with suitable material from the excavation, free from rock, lumps, wood, or other foreign matter, unless otherwise shown or ordered. 3.4.3 Backfilling around footings shall be done immediately after form removal. Backfilling adjacent to and over arch rings or rigid frame structures shall not be performed until 21 days after the concrete has been placed, or the test cylinders prepared from the fresh concrete and cured in the laboratory have attained 80 percent of the minimum compressive strength of the class of concrete shown on the plans. Backfill material shall not exceed the optimum moisture content by more than 2 percentage points. The material shall be placed in layers not more than 12” loose depth and compacted. To avoid unequal pressures, the material shall be